Distorted guitars through high-powered amplifiers are a bassists worst enemy. If you really want to overpower their amps, I don't think even a kilowatt will be enough. If you want to be audible and compete with them, a rule of thumb is 3-5 times their power, with the largest speaker array you can handle.

Of course, this all depends on how loud they are playing. If they don't turn the knobs to more than 3 or 4, 200 watts could do. If they insist on cranking them to 10, look for another band and let them destroy their hearing.

For people who need much power per dollar, I usually recommend Peavey and Carvin amps (although I don't really like either brand, but hey, that's just me and you can't beat their value).

And don't scoop your midrange (a very common "fault" people do) - it's the midrange that cuts through!

Oysterman is right 3-4 times the power will give you a clean power punch, less then that when you are on stage set your amp back to have your sound hit where you are or you will have a hard time hearing yourself,

Example: He has a Fender Hot Rod Deluxe with the 12" replaced by a JBL E-120 speaker. This is a 40 watt amp, and the E-120 puts out 103 SPL. You have an Acme B4 Series-II rated at 96 SPL attached to your bass head.

At a distance of 8 meters, the Fender will put out 100 SPL at 32 watts. You'll need at least 128 watts for you Acme B4's to keep up with him. If your speakers are very inefficient (like mine), you will need 1,000 watts to make the same noise he makes with 32 watts and a JBL.